Membrane potential is an electrical potential difference across an impermeable lipid bilayer (reviewed in (Hille (2001))). Membrane potentials occur across the plasma membrane of most living prokaryotic and eukaryotic cells, as well as across the membranes of some organelles such as chloroplasts and mitochondria. The cell's membrane potential results from the active translocation of ions (by ‘pumps’) between the extracellular fluid and the cytoplasm, generating electrochemical concentration gradients along which these ions then have the potential to flow. By selectively regulating the flow of ions (often sodium and potassium) down these gradients, ion channels are the primary effectors of electrophysiological events that mediate cell signaling, development, differentiation of function, and pathology. The membrane potential of a typical resting cell is −70 mV (the cytoplasm being electrically negative with respect to the extracellular fluid). Changes in this membrane potential are usually effected by the coordinated action of many types of ion channels. Changes in membrane potential in the positive direction (i.e. resulting in a net positive intracellular charge) are referred to as depolarization. Changes in the negative direction (i.e. resulting in a net negative intracellular charge) are referred to as hyperpolarization.
Ion channels are membrane-bound proteins that control the flow of ions across biological membranes. There are about 400 recognized ion channel genes in the human genome (Venter, et al. (2001), Science, 291:1304-51), encoding proteins specific for potassium (K+), sodium (Na+), chloride (Cl−), calcium (Ca2+), and hydrogen (H+) translocation. Many ion channels are capable of transporting more than one ion species. Potassium ion (K+) channels have been particularly well studied due to their primary importance in excitable cells (Ford, et al. (2002), Prog Drug Res, 58:133-168)). A number of toxins are known to bind specifically to K-channels, a handful of K-channels have been crystallized, and a variety of K-channels with various regulatory features have been defined and characterized.
Ion channels are responsible for a variety of essential cellular processes throughout the body, most notably (although by no means exclusively) the generation of action potential in excitable tissues (muscle and nerve). In cardiac ventricular cells, for example, depolarization is achieved by the inward-translocation of Na+ by cardiac sodium channels (e.g. SCN5a), while hyperpolarization involves a diverse number of potassium channels, including the KCNQ1 and hERG K+ channels. The dysfunction of ion channels is responsible for many common disorders such as cardiac arrhythmias, neurological and behavioral diseases, diabetes, hypertension, angina pectoris, and cystic fibrosis. In addition to being important molecular targets for treating human disease, ion channels are also a major cause of off-target adverse effects and poor drug safety. For example, more than 50% of all drug withdrawals from the market since 1998 have been due to adverse and unpredicted effects on the functions of the hERG K+ channel. For these reasons, high-throughput systems that facilitate the screening of compounds against ion channels are of significant interest to the pharmaceutical industry, both for monitoring drug safety as well as for developing disease therapeutics.
About 15% of the top 100 best-selling drugs target ion channels. New drugs that target ion channels are therefore highly sought after by pharmaceutical companies. However, traditional methods of discovering ion channel drugs have consistently failed to identify novel ion channel inhibitors. Ion channels therefore remain one of the most promising, but underexploited, molecular targets in drug discovery.
Among the most important ion channels needed for High Throughput Screening (“HTS”) are those involved in the pathogenesis of cardiac arrhythmias. More than 300,000 deaths each year are attributed to ventricular tachyarrhythmia, the most dangerous form of cardiac arrhythmia. Among the ion channels important for regulating the ventricular action potential are KCNQ1, hERG, SCN5a, and their beta-subunits minK and MiRP1 (reviewed in (Keating, et al. (2001), Cell, 104:569-80)). Collectively, these five proteins are involved in the pathogenesis of over 95% of inherited cardiac arrhythmias (including long QT syndrome (LQT), familial ventricular fibrillation, and Jervell and Lange-Nielsen syndrome). KCNQ1 and hERG alone are responsible for 87% of inherited LQT. Although familial LQT is relatively rare, linkage of these genes to LQT validates them as targets to treat acquired forms of the disease.
Perhaps of broader significance, off-target inhibition of the IKr current (hERG+MiRP1) is responsible for nearly all drug-induced LQT (Keating, et al. (2001), Cell, 104:569-80), and has been responsible for 50% of all drug withdrawals from the market since 1998. Cross-screening for hERG reactivity is now highly recommended by the FDA for all drugs, and is being integrated into the drug-development process of most pharmaceutical companies.
Many FDA-approved drugs that inhibit ion channels preferentially bind open or inactivated states of their target channels, often with 1-2 orders of magnitude greater affinity. Drug targeting to particular states of an ion channel can involve open-state block, inactivated-state block, voltage-dependent block, and use-dependent block (reviewed in (Birch, et al. (2004), Drug Discov Today, 9:410-418)). The “open” state of an ion channel also includes certain inactivated states in which the channel is in a non-conducting but open conformation. Examples include nearly all drugs that exert effects on the hERG K+ channel (Kamiya, et al. (2001), Mol Pharmacol, 60:244-53, Mitcheson, et al. (2000), Proc Natl Acad Sci USA, 97:12329-33, Sanguinetti, et al. (2005), Trends Pharmacol Sci, 26:119-24), most Nav-channel drugs (particularly anti-convulsants and anesthetics) (Yang, et al. (2002), Mol Pharmacol, 62:1228-37), the alkyl ammonium class of ion channel inhibitors (e.g. TEA), and a large number of Ca2+ channel modulators. However, most ion channels presented by cells during a typical HTS screen are in the closed state (in a typical cell-based HTS, the channels are simply expressed on the cell surface, a resting state in which most ion channels will be closed). Thus, potential inhibitors that require an open state to bind may not be easily detected in a typical cell-based screen. The ability to screen for compounds that preferentially bind different ion channel conformations could greatly facilitate the identification of novel drugs.
The 97 amino acid influenza A M2 protein is the best-characterized of a growing number of putative ion channels encoded by different viruses (reviewed in (Kelly, et al. (2003), FEBS Lett, 552:61-7). This pH-activated H+ ion channel functions as a homotetramer, its gating properties and specificity have been well-studied, and it is an established determinant of influenza pathogenicity and infectivity (reviewed in (Fischer, et al. (2002), Biochim Biophys Acta, 1561:27-45, Pinto, et al. (2004), FEBS Lett, 560:1-2)). M2 is expressed in large quantities on the plasma membrane of infected cells, from where it is incorporated into the viral membrane during virion budding. As influenza particles enter new target cells by endocytosis, they become exposed to low pH within endosomes. The low pH triggers M2 gating which is believed to allow H+ to enter the virus and lower the internal viral pH. Acidification of the virion interior is necessary to facilitate release of viral nucleic acid and nucleocapsid protein (together, the ‘RNP’ complex) from the M1 structural protein, a pre-requisite for nuclear migration of the viral genome. The M2 protein also has a separate function late in viral assembly, neutralizing Golgi pH to ensure that the viral envelope (HA) is not prematurely activated.
M2 is conserved in all strains of influenza, and artificially engineered M2-deficient strains are attenuated (Takeda, et al. (2002), J Virol, 76:1391-9, Watanabe, et al. (2001), J Virol, 75:5656-62), indicating the vital role of this protein in viral pathogenicity. Two of the four FDA-approved influenza therapeutics target the M2 ion channel, validating it as a molecular target for the treatment of influenza infection. FDA-approved influenza drugs (non-vaccines) include amantadine (α-M2, 1976), rimantadine (α-M2, 1993), zanamivir (α-NA, 1999), and oseltamivir (α-NA, 1999). Amantadine, and the structurally-related rimantadine, inhibit viral entry and assembly, and are the only FDA-approved influenza drugs for treatment and prophylaxis. Influenza B and C possess M2 proteins that likely play similar roles to that of influenza A M2, but these targets are not yet completely validated (in part because inhibitors do not yet exist).
The need for new, broad-spectrum inhibitors of M2 is urgent, as influenza strains resistant to existing M2 drugs are becoming increasingly prevalent (Abed, et al. (2005), Antimicrob Agents Chemother, 49:556-9). However, no new M2 drugs have been developed in over a decade (and the last, rimantadine, is a structural derivative of amantadine), in part due to the difficulty of screening for new M2 inhibitors. Ion channels in general are among the most difficult proteins to manipulate for HTS. The primary strategy for detecting M2 ion channel activity and its inhibition employs patch clamp of cells (mammalian or oocyte). Both the patch clamp technique and the use of cell expression vehicles are poorly suited for HTS of M2 inhibitors.
A number of other putative viral ion channels, including influenza B M2 and NB, influenza C M2, HIV-1 Vpu, and Vpr, Chlorella Kcv, Paramyxovirus SH, and rhinovirus 3AB, possess potential ion conducting activities (reviewed in (Fischer, et al. (2002), Biochim Biophys Acta, 1561:27-45, Lamb, et al. (1997), Virology, 229:1-11)). However, their contributions to viral entry and replication are largely unknown. A confluence of indirect evidence supports the role of some of these proteins (particularly BM2, CM2, and Vpu) in the viral lifecycle, but has been inconclusive, unavailable, or contradictory regarding others. For example, the NB protein is similar in some respects to M2 (NB is a 100 amino acid single-TM protein incorporated into influenza B virions that can, if reconstituted into lipid bilayers, demonstrate ion conducting activity (Fischer, et al. (2001), Eur Biophys J, 30:416-20, Sunstrom, et al. (1996), J Membr Biol, 150:127-32)), but its function as an ion channel within the virus has been highly controversial (Lamb, et al. (1997), Virology, 229:1-11, Mould, et al. (2003), Dev Cell, 5:175-84). The uncertainties of indirect measurement have hindered the development of viral ion channels as targets for the control of infection, and primarily exist because methods of studying the function of viral ion channels directly within their native virus structures have been unavailable. New methods for identifying viral ion channels and understanding their role in viral infection could enable a number of new questions to be asked about the function of the proteins within the viral structure and allow new possibilities for anti-viral drug development.
The traditional method for measuring membrane potential involves directly inserting glass microelectrodes into or onto isolated cells (patch clamp) to directly measure the difference in electrical potential across the cell membrane. Although it is a very powerful technique, this is usually a cumbersome and low-throughput approach to monitoring ion channel activity. Recently, automated planar patch-clamp systems have increased the ability to perform patch-clamps from about a dozen per day by hand to several hundred or several thousand by machine (Xu, et al. (2003), Assay Drug Dev Technol, 1:675-84). Despite its advances, however, planar patch-clamp (50-3,000 assays per day with an assay failure rate of >25%) is not being used for high-throughput screening (current high-throughput requirements are 100,000+ screens per day with a failure rate of <<1%). Automated patch-clamp systems are also expensive and require specialized technical expertise and attention. In addition, taking measurements from single cells increases error due to biological variation, resulting in unacceptable variability compared to required HTS standards (Sorota, et al. (2005), Assay Drug Dev Technol, 3:47-57). For these reasons, planar patch-clamp is primarily being used as a secondary screen for validating and characterizing lead compounds discovered by other techniques.
A high-throughput alternative to direct electrophysiological measurement techniques is the use of fluorescent probes that respond to changes in electrical membrane potential (reviewed in (Haugland (2003), Plasek, et al. (1996), J of Photochemistry and Photobiology, 33:101-124, Smith (1990), Biochim Biophys Acta, 1016:1-28)). Membrane potential-sensitive fluorescent probes are lipophilic dyes that associate with the lipid bilayer and change their spectral properties in response to membrane potential changes across that bilayer. The dyes are classified either as “slow” or “fast” depending on how they respond to the re-distribution of charges across the membrane. Slow dyes migrate to the opposite side of the membrane over seconds or minutes, while fast dyes respond within milliseconds by flipping their orientation within the membrane. Membrane potential-sensitive fluorescent probes were first employed in the 1970s, and have since evolved in capability, speed of response, and sensitivity (Cohen, et al. (1978), Rev Physiol Biochem Pharmacol, 83:35-88, Plasek, et al. (1996), J of Photochemistry and Photobiology, 33:101-124, Smith (1990), Biochim Biophys Acta, 1016:1-28, Wolff, et al. (2003), J Biomol Screen, 8:533-43, Zochowski, et al. (2000), Biol Bull, 198:1-21). For example, many membrane potential probes can now be measured “ratiometrically” to provide a more stable baseline (independent of dye loading and leakage) and to increase signal-to-noise values (up to 10-fold compared with single wavelength measurements) (Haugland (2003), Montana, et al. (1989), Biochemistry, 28:4536-4539). Many lipophilic membrane potential dyes partition quickly into lipid bilayer membranes, and have little or no fluorescence in aqueous solution (and therefore excess dye need not be washed away after loading into membranes). Membrane potential-sensitive dyes have been used to monitor membrane potential in eukaryotic cells, prokaryotic cells, and lipid vesicles, but have not been used previously to measure membrane potential in viral structures. Several popular commercial devices rely on membrane potential probes to enable high-throughput screening of ion channel activity within cells, including Panvera/Invitrogen's VIPR platform and Molecular Device's FLIPR system (Gonzalez, et al. (1997), Chem Biol, 4:269-77, Larson (2003), Discovery HTS, 1:5-6, Whiteaker, et al. (2001), J Biomol Screen, 6:305-12).
There are many examples of membrane potential dyes. Molecular Devices' FMP dye, consists of the DiSBAC(1)3 voltage-sensitive dye combined with a quencher, Direct Blue 71, that improves signal-to-noise (Baxter, et al. (2002), J Biomol Screen, 7:79-85, Whiteaker, et al. (2001), J Biomol Screen, 6:305-12, Wolff, et al. (2003), J Biomol Screen, 8:533-43)(U.S. Pat. No. 6,852,504) The commercially available FMP dye has been optimized for cellular applications. For many years, DiSC3(5) was considered to be the gold-standard for membrane potential assays due its high sensitivity (50-80% per 100 mV, the highest of all cyanine dyes, (Plasek, et al. (1996), J of Photochemistry and Photobiology, 33:101-124)). However, this high signal is achieved by a concomitant high accumulation of the dye in cells that reduces cell health and can cause cell death, precluding its use for many applications. Its high sensitivity, its ability to be measured ratiometrically (Haugland (2003), Montana, et al. (1989), Biochemistry, 28:4536-4539), and the lack of toxicity restrictions in lipoparticles, make DiSC3(5) an excellent choice for use in lipoparticles. RH 421 has the highest sensitivity of the “fast” membrane potential probes, greater than 20% fluorescence change per 100 mV (Haugland (2003)), making it an excellent choice. Its use has been restricted by cellular toxicity and cell type variability, both of which are overcome using lipoparticles. The ANEPPS dyes (di-4-ANEPPS, di-8-ANEPPS, and related derivatives) exhibit good photostability, a very fast response (<1 msec), and a uniform 10% change in fluorescence intensity per 100 mV (Haugland (2003), Rohr, et al. (1994), Biophysical Journal, 67:1301-1315, Zochowski, et al. (2000), Biol Bull, 198:1-21). The dye is essentially nonfluorescent in aqueous solution but has an absorption/emission maxima of 467/631 nm, which can be measured ratiometrically (dual excitation at 440/530).
Membrane proteins, including ion channels and G protein-coupled receptors (GPCRs), can be incorporated into viruses and virus-like particles (VLPs). We have previously incorporated both ion channels and GPCRs into MLV-based VLPs ((Doranz, et al. (2004)) U.S. patent application Ser. No. 10/901,399). Similarly, GPCRs have been incorporated into baculovirus and used for various assays (Bouvier, et al. (1998), Curr Opin Biotechnol, 9:522-7, Klaassen, et al. (1999), Biochem J, 342 (Pt 2):293-300, Loisel, et al. (1997), Nat Biotechnol, 15:1300-4, Masuda, et al. (2003), J Biol Chem, 278:24552-62).
Thus there is a need for methods and compositions to study ion channels and for screens to identify compounds that modulate the activity of ion channels. The present invention satisfies these needs as well as others.